It is known to use relay couplers or optical couplers for issuing a message or an indication on the error indicating line when an error occurs. Such couplers also serve to carry out galvanic separation of different parts of the circuit configuration. In the case of optical couplers, a power supply capable of functioning is absolutely required. In this previously known state of the art, the error message is issued to the error indicating line as a static signal, i.e. the potential of the error indicating line is changed between a first value and a second value when an error occurs, and remains at this second value for the duration of the error. It is disadvantageous, in this connection, that the error indication does not allow any statement concerning the type of error, so that it was not possible to evaluate whether this is a serious error and what the reason for the occurrence of the error might be.